1. Field of the Invention
This invention relates to a magnetic recording circuit for storing information on a magnetic recording medium such as a floppy disk.
2. Description of the Related Art
As advances have been made in recent years in computer technology, the importance of magnetic recording and reproducing devices such as a floppy disk and a hard disk, as an external storage means of a computer, are on the rise. For instance, these magnetic recording means have been widely used also in office automation equipments such as a personal computer, a microcomputer, a word processor, etc.
In general, in recording information on a floppy disk, a magnetic head is brought in intimate contact with the magnetic recording surface of a floppy disk rotating at a constant speed, and a write/read coil built in the magnetic head is excited to be operative as a current flows in the coil.
The magnetic recording method will now be described in greater detail with reference to FIGS. 2 and 3 of the accompanying drawings.
A floppy disk 10 is donut-shaped and stores information according to the magnetizing direction of a magnetic substance layer formed on the surface of the disk. The recording portion in the floppy disk 10 is divided into a predetermined number of tracks, e.g., about eighty annular tracks each subdivided into a plurality of circumferential sectors. Storage of information is performed according to the magnetizing directions one for each of predetermined sections in the individual sectors.
When data of "0" and "1" shown in FIG. 3(A) are to be stored, the data are first converted into a signal shown in FIG. 3(B), for example. This type of modulation system is called an FM modulation system. If there is a pulse signal between adjacent predetermined clock signals, it means "1"; and if there is no pulse signal between adjacent clock signals, then it means "0". In accordance with this signal, magnetization in the pattern such as shown in FIG. 3(C) is taken effect on the floppy disk. The modulation system shown in FIGS. 3(D) and 3(E) is an MFM modulation system, in which part of a clock signal is omitted.
Thus storage f information on the floppy disk 10 is taken effect by magnetizing the magnetic substance according to a predetermined system. Since the floppy disk 10 is doughnut-shaped as shown in FIG. 2, the areas of the individual tracks increase progressively from the inside toward the outside. Consequently the area to be magnetized for recording a datum is larger in an outer track than in an inner track, which necessitates increasing the magnetic flux density higher in an outer track than in an inner track when magnetizing.
To this end, it has hitherto been a common practice to divide the recording portion of the floppy disk 10 into an inner region (a) and an outer region (b), and in storing data in the outer region (b), to increase the quantity of current to be supplied to the write coil, thereby increasing the magnetic flux density.
FIG. 4 illustrates an example of the abovedescribed prior art magnetic recording circuit. In FIG. 4, by a current to flow in the write/read coil 20, a predetermined magnetizing treatment is taken effected on the surface of the floppy disk located in intimate contact with this coil 20. Apart from a certain external parts, these circuits are integrated on a single semiconductor chip.
Centrally in the coil 20 outside the integrated circuit, a center tap connected to a power source V.sub.cc is located and is connected at opposite ends to a pair of differential transistors 22, 24 via a pair of pins P, P, respectively. The emitters of these two differential transistors 22, 24 are connected to the ground via a common current-limiting transistor 26. To the bases of the differential transistors 22, 24, signals from a recording control circuit 28 are introduced. If any one of the differential transistors 22, 24 is rendered conductive, a current determined by the transistor 26 flows in the coil 20 in a predetermined direction.
Meanwhile, the quantity of current to flow to the transistor 26 is determined by the quantity of current to flow to the transistor 29 constituting a current mirror. Here a transistor 30 is a buffer transistor for constituting a current mirror. Further, the potential at the collector side of the transistor 29 is kept at a predetermined level by a constant voltage circuit 31 in the integrated circuit.
The quantity of current to flow to this transistor 29 is determined by a pair of exterior resistors 32, 34. That is, since a switching transistor 36 is connected to the parallel resistor 34 in series, the resistors 32, 34 are interconnected in parallel when the switching transistor 36 assumes an operative state. When the switching transistor 36 assumes an inoperative state, the resistor 34 is separated from the circuit so that a current flows to the transistor 29 only via the resistor 32. Therefore the quantity of current to flow to the transistor 29, namely, the write current to flow to the write coil 20 can be controlled by selectively bringing the switching transistor 36 into an operative state or inoperative state. Designated by 38 are a pair of diodes which serve to prevent any reverse current flow.
Thus by switching the transistor 36 between operative and inoperative, it is possible to control the quantity of write current in the inner region (a) and the outer region (b), individually, of the floppy disk 10 (FIG. 2). Designated by 39 is a reproducing circuit for reading and reproducing the information recorded on the floppy disk.
Also in the conventional magnetic recording circuit, it is known to take a change-over of the exciting current between two values depending on the outer and inner tracks of a floppy disk. Because the area of each track is different from that of any other track, it is preferred to take a change-over of the write current for each and every track.
However, with the conventional arrangement, such a fine change-over of the write current was not considered. Yet assuming that it could be realized, since this current value was adjusted by the external resistors, it should have increased the number of kinds of external resistors to vary the value of an exciting current in an effort to meet with the individual tracks exactly. Further, since the resistors are exterior, it is necessary to use as many signal input terminals as the number of steps to be changed over increases, thus enabling only an inadequate change-over of the write current.